1. Field of the Invention
The field of this invention relates to devices which shift signals from one frequency to another. In particular, this mixer operates with frequencies in the microwave region.
2. Description of the Prior Art
In virtually all communications systems it is necessary at some point to shift a signal, without distortion, from one frequency to another frequency. The component used to perform this frequency shifting operation is called a mixer. Virtually all microwave mixers are realized as one of two fundamental types: (1) a passive Schottky-barrier diode mixer, or (2) an active MESFET mixer. In the Schottky-barrier diode mixer; a signal called the local oscillator (LO), is applied along with the input signal, the RF, to a Schottky-barrier diode. Because of the nonlinear current/voltage characteristic of the diode, a third frequency, the IF, is generated at a frequency equal to the difference between the RF and LO frequencies, but is otherwise identical to the RF signal. Thus, this "mixing" operation between the RF and LO in effect shifts the RF to the IF frequency.
In the active MESFET mixer, the nonlinear transfer characteristic between the gate voltage and the drain current of a GaAs MESFET (Metal Epitaxial Field Effect Transistor) is used to achieve the same mixing operation. The RF and LO signals are applied to the gate of the MESFET, and the drain current includes the IF signal. The advantage of an active MESFET mixer over a diode mixer is that it can amplify the RF input signal as well as shift its frequency; it is, however, a more expensive component because the GaAs MESFET is generally more expensive than a Schottky-barrier diode.
A property common to both the active MESFET and Schottky-barrier diode mixers is that they make use of the nonlinearities in solid-state devices in order to achieve frequency mixing. However, those same nonlinearities create intermodulation distortion, and give rise to spurious responses, a closely related phenomenon. Thus, there is a fundamental conflict between efficient mixing and distortion in these mixers: strong nonlinearities are essential in order to obtain efficient mixing, but those same nonlinearities also generate high levels of distortion.
Several methods have been employed to reduce intermodulation distortion in mixers. The most common is to use a balanced configuration, which partially rejects certain intermodulation products. However, a balanced mixer has several shortcomings: (1) the balanced configuration does not even partially reject the most serious manifestations of intermodulation distortion (the so-called third-order intermodulation products); (2) the use of a balanced configuration increases the required LO power by a factor of two to eight; and (3) active MESFET mixers are not easily realized in balanced configurations.
The only other effective method for reducing intermodulation distortion is to use very high LO power levels. However, these power levels are difficult and often expensive to achieve in high-frequency mixers, especially if they are balanced. Also, only a limited amount of LO power can be applied without damaging the diodes or MESFETs in the mixer.
It is therefore an object of the present invention to provide a mixer that exhibits superior intermodulation performance at reasonable LO power levels without the reduction of other performance parameters.